Portable electronic devices, such as laptops computers, smart phones and tablets, are powered by portable batteries when not connected to another power source. Typical battery voltages on mobile devices are much greater than the operating voltages for the various chip components. Typical batteries have voltage ratings in the range of 6-12 volts. Modern processors and electronics included in portable electronic devices operate at about 1 volt or less. DC-to-DC power converters are employed to reduce the battery voltage to useable levels for different components around the chip. The required step-down of voltage should also maximize battery life.
DC-DC conversion is primarily performed today using either linear regulators, switched-inductor, or switched-capacitor converters. Linear regulators are small, yet inefficient when voltage conversion ratios are large. Switched-inductor converters can be very efficient, but require large inductors that can limit the thinness of smartphones and other mobile platforms. Decreasing the size of the inductor would improve the form factor and cost of such devices.
The growing demand for both performance and battery life in portable consumer electronics requires converter and power management circuits to be small, efficient, and dynamically powerful. Dynamic voltage scaling (DVS) can help achieve these goals in load circuits, though generally at the expense of increased DC-DC converter size (through use of external inductors) or loss (through linear regulation). While switched-capacitor (SC) DC-DC converters can offer conversion in small, fully-integrated form-factors, their efficiencies are only high at specific discrete ratios between the input and output voltages. See, e.g., D. El-Damak et al., “A 93% Efficiency Reconfigurable Switched-Capacitor DC-DC Converter using On-Chip Ferroelectric Capacitors,” ISSCC Dig. Tech. Papers, pp. 374-375, February 2013; H.-P. Le et al., “A Sub-ns Response Fully Integrated Battery-Connected Switched-Capacitor Voltage Regulator Delivering 0.19 W/mm2 at 73% Efficiency,” ISSCC Dig. Tech. Papers, pp. 372-373, February 2013; Y. K. Ramadass et al., “A 0.16 mm2 Completely On-Chip Switched-Capacitor DC-DC Converter Using Digital Capacitance Modulation for LDO Replacement in 45 nm CMOS,” ISSCC Dig. Tech. Papers, pp. 208-209, February 2010; Y. K. Ramadass et al., “Voltage Scalable Switched Capacitor DC-DC Converter for Ultra-Low-Power On-Chip Applications,” IEEE Power Electronics Specialists Conference, pp. 2353-2359, June 2007; Y. K. Ramadass et al., “Voltage Scalable Switched Capacitor DC-DC Converter for Ultra-Low-Power On-Chip Applications,” IEEE Power Electronics Specialists Conference, pp. 2353-2359, June 2007. More conversion ratios can increase the voltage output resolution. However, the additional ratios can escalate the number of components with employment of conventional topologies. As a result, many such converters only employ a small number of conversion ratios.
A successive approximation (SAR) SC topology has been proposed to address the dilemma of increased numbers of ratios causing unacceptable increases in complexity. See, S. Bang et al., “A Fully-Integrated Successive-Approximation Switched-Capacitor DC-DC Converter with 31 mV Output Voltage Resolution,” ISSCC Dig. Tech. Papers, pp. 370-371, February 2013. This topology cascades several 2:1 SC stages to provide a large number of conversion ratios with minimal hardware overhead. However, the linear cascading of stages introduces cascaded losses, limiting overall efficiency. For example, the minimum Rout is more than 30×Rout of a similar ratio Series-Parallel topology using the same silicon area. Additionally, current density is limited to that of a single stage, and capacitance utilization can be low for many of the conversion ratios.
Conventional commercial DC-DC Voltage Regulator IC's (VRICs) are a specific type of DC-DC switched inductor converter often employed for the DC-DC stepdown from battery voltage to portable device voltage levels. These converters tend to be bulky. These voltage regulator chips occupy a disproportionately large area, far more chip space than their simple role should warrant. The chips are also manufactured via a separate process from the remaining circuitry.
Because of the integration issue, the present state-of-the-art voltage regulation solution uses a separate on-board VRIC to power each component in the electronic system. A typical number of VRICs ranges from 20 in Notebooks to 7 ICs in devices like Bluetooth handsets. VRICs can occupy a significant percentage of the platform of a device, e.g. ˜38% of the platform area in a popular ultra-small notebook computer.
Mobile electronic systems powered by Li-ion batteries typically employ a power management integrated circuit (PMIC) to stepdown the 2.8-4.2V battery voltage to a voltage more appropriate for load circuits (e.g., 0.5-1.8V). Most PMICs employ a switching inductor (SL) architecture, utilizing an off-chip inductor as a vehicle for energy conversion. While such designs can offer high efficiency over a wide range of voltages, they suffer from high cost and large area. In contrast, switched-capacitor (SC) DC/DC converters utilize capacitors that can have significantly higher power densities, 7× lower BOM cost, and 8× smaller footprint than typical power inductors. However, SC converters are only efficient at discrete ratios of input-to-output, and increasing the number of ratios with conventional topologies requires exponentially more capacitors and as a result a larger PCB footprint. Even recent work that achieves many ratios in modular topologies still requires large number of capacitors at large conversion ratios [L. Salem and P. P. Mercier, “An 85%-Efficiency Fully-Integrated 15-Ratio Recursive Switched-Capacitor DC-DC Converter With 0.1-2.2V Output Voltage Range,” ISSCC, February 2014; L. Salem and P. P. Mercier, “A 45-Ratio Recursively Sliced Series-Parallel Switched-Capacitor DC-DC Converter Achieving 86% Efficiency,” CICC, September 2014] and thus SL converters are currently the standard choice for PMIC designs.
Switched-capacitor (SC) converters can offer high efficiency and small size, yet have difficulty achieving these specifications at high power density, in part due to fundamental charge-sharing losses. Resonant SC converters (ReSC) have been proposed that combine inductors with capacitors to achieve high efficiency and power density, all in a small size. However, known ReSC converters typically require one inductor per flying capacitor to achieve resonant (or soft-charging) operation of all underlying capacitors. This limits the achievable number of DC-DC conversion ratios, thereby limiting the utility of ReSC converters in practical applications.
Seeman, U.S. Pat. No. 8,368,369 discloses a voltage regulator that provides hysteretic regulation of switched-capactor converters. The regulator uses a set of cascaded flip-flops corresponding to phases of the converter. The set of cascaded flip-flops has a plurality of clock inputs coupled to the comparator output. The converters are phase interleaved converters with ratio configuration logic. The set of cascaded flip-flops have a plurality of clock inputs coupled to the comparator output, and a plurality of phase outputs trigger a phase transition in the converter if the output voltage signal falls below the reference voltage signal.
The commercial standard used for switched capacitors by major manufacturers is exemplified by the Texas Instruments TPS6050x step-down charge pumps. The gear logic uses a comparator and a resistor per selectable conversion ratio.
Recent Salem US Published Patent Application 2014/0043010 discloses an advance in DC-DC converters. The application discloses DC-DC converters that are capable of operating at one of a plurality of voltage conversion ratios. The converters include a plurality of switched cells that can be connected in a cascade, stack or a cascade and stack. Each cell can operation to provide a 2:1 transformation (voltage conversion level) to provide an output that is ½ the input to the cell. This new type of converter did not specify new control strategies to select the particular ratios. Recent Salem et al US Published Application 20140184189 discloses an inductively assisted switched capacitor DC-DC converter. The converter includes plurality of inductors that provide continuous modes from the plurality of distinct ratios obtained in a capacitive circuit.